1. Overview
The core algorithm of Smart antenna is adaptive beamforming antenna technology which has been developed at 1960s, used in radar antenna array, in order to enhance the performance of radar and electronic warfare capabilities. In the mid-1990s, smart antenna technology began to be considered in wireless communication systems. Firstly United States corp. Arraycom implemented the smart antenna in the Time Division Multiple Access PHS system;
In 1997, Beijing Xinwei Telecom Technology Co., Ltd. Which companies from China Ministry of Information Industry Academy of Telecommunications Technology has successfully development SCDMA wireless subscriber loop System based on smart antenna technology, In addition, a large number of universities and research institutions of china and other country have begun to carry out a wide range of smart antenna beamforming algorithm.
In year 1998, China submitted TD-SCDMA RTT proposal to the ITU's, which is the CDMA communication system first time based on smart antenna technology.
2. The basic principles of Smart Antenna (SA)
Principle of Smart antenna technology is to make a set of antenna elements arranged in certain shape, the signal incentive into corresponding transceiver, based on wave interference principle, we can have a strong directional radiation pattern.
If using base band digital signal processing methods, making the radiation pattern of the main valve to adaptive point to the user’s direction of arrival (DOA), we can enhance the signal to interference rate (SNR), reduce the transmitting power and increase the coverage of the wireless system.
(1) The smart antenna element is usually offset by a straight line, circle or isometric plane order. Each element is Omni-directional antenna.
(2) When the mobile station is far enough away from the antenna, the actual incidence of the signals can be considered as coming from same direction.
Smart Antenna System includes the following main components: ① smart antenna array; ② front-end modules of RF Unit (including linear power amplifiers, low noise and control / monitoring circuit); ③ RF band pass filter; ④ cable systems (RF cables, Control cables, RF lightning protection modules, low-frequency lightning protection circuit). See Figure 1.
Figure 1 structure diagram of Smart antenna
See Figure 1, if antenna arrays include M elements that arranged in a straight line and equidistant distance, then the distance difference between the incidents is Δd = m • Δx • cosθ, the phase difference of time-domain incident is (2 • pi/λ) • Δd.
Figure 2 Received signal of Smart antenna
Therefore the space arrived distance difference led to the receiving signals at different phase for each element. After multiply by weighted array, the output signals are as follows:
Where A is the constant signal amplitude, s(t) is the complex envelope of the signal and wm is the array weights.
Assuming that the weighted factor of m - array element is 
According to the superposition of the sine wave result, then:
If we choose a different Φ0, the point direction of beamforming will be changed, So we can change the array weights value in order to choose the right direction.
